KINETICS OF SURFACE PROCESSES FOR MO(CO)(6) ON PARTIALLY DEHYDROXYLATED ALUMINA AND HYDROXYLATED ALUMINA - OBSERVATION OF MO(CO)(5)(ADS)

The adsorption of Mo(CO)(6) on partially dehydroxylated alumina (PDA) and hydroxylated alumina (HA) has been studied using IR and UV-vis spectroscopy. The results from these experiments suggest that the initially physisorbed Mo(CO)6 coordinates to two distinct Lewis acid sites on the surface of PDA, one much more abundant than the other, with an apparent single rate constant 2.3 x 10(-3) s(-1) at 298 K. The Mo(CO)(6)(ads) in turn loses CO reversibly, with an apparent single rate constant 1.8 x 10(-4) s(-1) at 298 K to form Mo(CO)(5)(ads). Upon removal of gas phase CO released in the first step, Mo(CO)(5)(ads) loses two additional COs to form Mo(CO)(3)(ads). Alternatively, on HA physisorbed Mo(CO)(6) undergoes nucleophilic attack by hydroxyl groups, which results in cis-labilization of a carbonyl group, leading in turn to the formation of Mo(CO)(5)(L), where L is a surface hydroxyl. The Mo(CO)(5)(L) so formed loses additional carbonyls to form a lower subcarbonyl. The decarbonylation process appears to be faster than on PDA. The experimental data indicate that there are no Al3+ exposed on HA. All the observed decarbonylation processes are reversible under CO at room temperature on both HA and PDA. The addition of CO2 to the subcarbonyl on HA results in the formation of a bicarbonate, with displacement of the subcarbonyls.